OK
Vagina is a medical word coined the 1680's. It comes from the Latin vagina meaning a scabbard or sheath. So I think we can assume the plural has always been vaginae, as I can't imagine a 17th century man of medicine anglicising the plural when at that time nearly all scientific and medical writings were written in classical Greek or Latin, English being considered a peasants language.
( The Awful Puppy Bakery is still bored, Mon 6 Dec 2010, 13:07, 1 reply, 15 years ago)

Now you mention it
scabbard is a far better word.
( sandettie light vessel automatic New Twitter - @bollocksreally, Mon 6 Dec 2010, 18:38, Reply)